The main goal of this project is to identify novel molecular targets for drug discovery in cancer therapy. We study the pathway deregulation that is specific to cancer cells (opposed to. normal cells), but common to most types of cancers in the hope to identify the Achilles' heel of cancer. Oncogenic stress can lead to activation of p53, which induces either apoptosis or cellular senescence and serves as an important barrier to tumorigenesis. Only when DNA damage response pathway such as p53 is inhibited, cells then develop into cancer. We have identified a key event during oncogenic activation that leads to inhibition of p53 function. Targeting this pathway might reconstitute the barrier to tumorigenesis or enhance chemotherapy efficacy. This proposal is to delineate the mechanistic details of this pathway and to establish a framework foundation for novel therapeutic approaches in cancer. TopBP1 (DNA topoisomerase II2 binding protein 1) contains eight BRCA1-carboxyl-terminal (BRCT) motifs, which are found in proteins involved in DNA repair and cell cycle checkpoint control. It is involved in DNA replication, ATR activation and transcriptional regulation. Consistent with a role in promoting growth and survival, TopBP1 is an E2F target, and its level rises at G1/S transition and S phase. Our preliminary studies demonstrate that TopBP1 is often overexpressed in cancer and its overexpression is associated with poor survival. More importantly, data accumulated in our lab demonstrate a role for TopBP1 in controlling two critical transcriptional factors E2F1 and p53, especially their pro-apoptotic activities. Loss of pRb tumor suppressor pathway leads to excessive E2F activities, which in turn activate TopBP1 expression. Elevated TopBP1 levels then cause inhibition of E2F1- and p53-dependent apoptosis. This event might contribute shorter patient survival in cancer harboring high levels of TopBP1. Thus, we hypothesize that TopBP1 could serve as a novel therapeutic target to potentiate both E2F1- and p53-mediated apoptosis in cancer therapy. To this end, we will carry out both in vitro and in vivo experiments to identify strategies in targeting the TopBP1 pathway for cancer therapy. Aim 1. Elucidate the mechanism and dynamic regulation of TopBP1 and p53 interaction. Aim 2. Investigate the mechanisms and biological outcomes of TopBP1 and mutant p53 interaction. Aim 3. Target a critical domain of TopBP1 to reactivate E2F1 and p53 and enhance chemosensitivity. Upon completion of this project, we will elucidate the TopBP1 pathway in cancer cell survival and define its molecular details and mechanisms. We will also identify the critical motif within TopBP1 for drug development and validate its target utility.